The invention relates to an apparatus for pointing spatial coordinates, comprising a movable hand-held probe, having a pointing tip, and a portable base unit provided with a rotatably supported elongated arm, wherein the hand-held probe connects to the portable base unit by means of a cord or a wire via the elongated arm and wherein the base unit is provided with sensors for measuring length or a change in length of the cord or the wire and rotation of the arm in at least one degree of freedom, and computer-controlled processing means for processing measuring signals delivered by said sensors into position data of the hand-held probe.
The spatial coordinates are understood to mean parameters defining the position in a space with respect to a reference point in either a two dimensional or three dimensional coordinate system. These parameters may be distance, azimuth angle, and elevation angle of the relevant point with respect to the reference point.
An apparatus of the above mentioned type is known from U.S. Pat. No. 6,785,973, which can be used for measuring the shape or contour of two-dimensional or three-dimensional objects, such as small objects to be placed on a measuring table, or relatively large objects disposed in a room.
The known apparatus is equipped with a cord or a wire, for connecting the movable hand-held probe to the portable base unit via an elongated arm. Therefore, only a single sensor suffices for determining the length or change in length of the cord or the wire. By using a cord or a wire, no strict limitations with regard to the length of the cord are imposed, and therefore it is possible to measure relatively large objects.
A second sensor in the apparatus is used in order to enable accurate determination of the angle or angular displacement of the cord caused by a change in position of the measuring probe. This second sensor, which is a rotation angle meter, is coupled to the rotatably supported arm in the longitudinal direction of which the cord or the wire engages the arm.
In an example, the sensors for measuring length or a change in length of the cord or the wire and rotation of the arm in at least one degree of freedom are understood to be at least one of a angle sensor, length sensor in the form of pulse generators, wherein the number of pulses delivered during use is proportional to a change in length or angular displacement of the cord or the wire or of the arm coupled thereto. Another option is to use potentiometers for measuring the rotation of the arm, and the change in length of the cord or the wire.
The computer-controlled processing means are arranged for processing the measuring signals delivered by the sensors, i.e. the angle or angular displacement of the cord and the length or change in length of the cord or the wire, into position data of the hand-held probe.
The positional accuracy of the hand-held prove of the above mentioned apparatus is determined by the sensor coupled to the rotatably supported arm and the sensor used for determining the length or change of length in the cord or wire.
It was the insight of the inventors to notice that the accuracy of the known apparatus is limited because the apparatus determines the positional data of the hand-held probe, which is not necessarily equal to the position of a desired point, for example the pointing tip of the hand-held probe. In the end, it is not the position of the hand-held probe, but for example, the position of the pointing tip of the hand-held probe which determines the accuracy of the apparatus for pointing spatial coordinates.
Consequently it is an object of the invention to provide an improved apparatus for pointing spatial coordinates, which determines the desired spatial coordinates, for example the actual position of the pointing tip of the hand-held probe.